Lens wheels (also known as lens-on-disc optical scanners) are used in a variety of optical systems for scanning a focused beam of light across a media to trace out data tracks. A lens wheel is generally comprised of a disk-shaped substrate having a plurality of focusing lenses positioned about the circumference of the disc. In an optical tape recorder system, for example, the lens wheel is rotated about its center such that a collimated beam of light, modulated by the data to be recorded and directed at the rotating wheel, successively illuminates each circumferentially moving lens. Each illuminated lens on the lens wheel focuses the modulated illuminating beam onto the recording media-to trace out one data track. Movement of the tape recording media in a direction perpendicular to the data track tracing action causes parallel data tracks to be traced onto the media by the rotating lens wheel to record the data. The lens wheel is also used to recover the recorded data by focusing an illuminating beam to read the data tracks previously recorded from the media with the recorded data modulating and reflecting the illuminating beam back through the rotating lens wheel to a detector.
Several different methods have been used to manufacture lens wheels. For example, one common method utilizes individual lens elements of identical design inserted and secured in openings around the circumference of a disk-shaped substrate. The drawback of this manufacturing method is that the optical performance of each lens in the wheel differs slightly even though the lens designs are identical. The difference in performance is due to the fact that there are differences in back focal distance between the lenses as installed in the disk. Furthermore, some lenses include aberrations or slight defects that, while within the design tolerance for the lenses, are not consistently present across all lenses. Thus, the write paths traced by each lens slightly differ, adversely affecting the performance of the optical system. In optical recording and playback applications involving very high data transmission rates, slight deviations in the traced write paths cannot be tolerated due to the close proximity with which the data tracks must be written to record high data volumes.
In order to increase the data storage capacity of the tape, not only must the data tracks be written close together, but the diameter of each recorded bit must be as small as possible. To achieve a small, high resolution bit diameter, the collimated beam illuminating each write lens on the rotating lens wheel must have a diameter large enough to fully illuminate the write lens as it moves across the recording media. The most common means used for fully illuminating successive write lenses on the rotating lens wheel is flood illumination of an area on the rotating lens wheel. The most common flood illumination pattern provides an elliptical illumination area having a major axis substantially corresponding to an arcuate portion of the circumferential path followed by each write lens while passing across the width of the recording media. With elliptical illumination, however, the illumination efficiency of the beam along the major axis corresponding to the arcuate path of movement of each lens across the media is not constant. Thus, complex and costly electronics are required for adjusting the illumination power of the beam as a function of lens wheel rotation to account for the differences in illumination efficiency as each write lens moves through the elliptical illumination area to trace out a data track. Furthermore, since the illumination area is much larger than the lens clear aperture, flood illumination is inefficient.
In view of the foregoing, there is a need for an improved lens wheel design and manufacturing method that will repeatedly produce lens wheels having similar performance characteristics. Furthermore, there is a need for a more efficient method for illuminating each successive write lens as the lens wheel rotates to trace out a data track.